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Issue
35 — October 2003 |
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35 |
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Titel |
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| Cover
Pictures |
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The
life cycle of a horny cell |
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The skin (cutis) is composed of two layers, the epidermis and the dermis
(corium). The outermost layer of the skin is the horny layer or stratum
corneum. It is composed of approximately twenty layers of brick-like, densely
packed horny cells which are interlocked with their neighboring cells. They
are formed by dead cells which are transformed into keratin (protein). These
cells do not contain any cell organelles.
The horny layer has the function to protect the living skin layers through
these numerous layers of flat, dead (horny) cells, which flake away at the
skin's surface. This process of sloughing off cells on the skin's surface
is called desquamation.
The horny cells, which, in this layer, have a length of approximately 30
µm and a thickness of 0.5 µm are baked together with the horny
substances (keratin) to form plates. These are finally sloughed off as horny
flakes. During the migration to the skin's surface the cells change their
appearance. This process is also called differentiation. In our illustration,
this process of differentiation is shown in various phases (see 1-4).
The epidermis consists of four cell layers. The cells of the epidermis migrate
through these layers within a life-time of about four weeks. The cells are built
by cell division in the lowest layer of the subcutis, the basal layer (1). The
cells change their appearance while they migrate to the skin's surface. They flatten
as they rise through the layers (2). In the stratum spinosum or prickle-cell layer
above the basal layer, large cells with "prickle-like" extensions are
found. In the stratum granulosum (or granular layer), which is located above the
prickle-cell layer, the cells become smaller (3), finally turn into dead cells
and are shed (4).
